Isobar
One of a set of atoms of different elements that share the same mass number but have different atomic numbers.
Formula / equation
same A, different Z
Unit
Dimensionless
In depth
Isobars have the same total nucleon count distributed differently between protons and neutrons, so their chemical behaviour is entirely different. Neighbouring isobars are frequently linked by beta decay, in which a neutron converts to a proton or vice versa while A stays fixed.
Examples in the real world
Argon-40, potassium-40 and calcium-40 all have A = 40 but Z = 18, 19 and 20 respectively.